From: gherbert@crl3.crl.com (George Herbert)
Newsgroups: sci.space.policy
Subject: Re: Challenger Disaster II
Date: 23 Oct 1999 23:44:21 -0700

Derek Lyons <elde@hurricane.net> wrote:
>[...]
>Metal (structural) fatigue is *the* determining factor in the lifespan
>of just about any piece of machinery.  So long as other parts can be
>replaced, and the actual structure is intact, there is no reason to
>mothball or decomission an aircraft, ship, etc....

A few objections.

First, things like aerodynamics improve over time, new jetliners
are a lot better optimized aerodynamically than the 707 was.
On ships, 40 years ago bow bulbs were not standard ship design,
they are now routine, so not even hydrodynamics is a static
field over the lifetime of vessels.  These small subtle changes
can by themselves economically obsolete vehicle hulls/airframes.

Second, fatigue is only a primary concern in aircraft.
Corrosion is a rough equivalent in ships, but can be nearly
100% prevented by careful design and maintenance.  Steel does
not exhibit the types of lifetime limits that Aluminum does
when you design it right.  Shipbuilding in particular requires
large margins (lavish by aircraft standards) over normal and
even extreme operating conditions, which means that ships do
not generally fatigue their structures.  When they do encounter
cracking it is a cause for extreme concern, rework, and analysis,
as it indicates not a minor but a major understrength or overstress
issue with the hull strength.  Minor cracks are normal in aviation
for long life aircraft, and crack detection and repair is a high
priority in aircraft heavy maintenance.

Thirdly, evolving technical and economic requirements often
obsolete airframes and hulls before they're worn out at all.
If I had a Comet today which had sat in a hangar being regularly
cleaned and greased, with 100 airframe hours and the strengthened
crack-proof airframe, the only customer would be a museum or scrapyard.
It's barely a trunkliner by current standards, has 4 hard to maintain
engines and atrociously complex and maintenance-prone hydraulics and
electrical systems by current standards.  It would not be economically
viable today, period, even if it was in excellent new or like new condition.

Fourth, the heavy systems and hull maintenance in an aircraft D-check
is only economically worthwhile because airframes are so expensive
and lightweight to start with.  With ships, tearing the whole thing
apart to re-wire, re-plumb and re-engine the whole ship is usually
more expensive than building a new one from scratch.


-george william herbert
gherbert@crl.com